Louvered film for unidirectional light from a point source

ABSTRACT

A process for producing film including radiation-opaque louverlike elements of divergent, or convergent, orientation. Pairs of such films for example, using radio-opaque louvers, are valuable as Bucky grids for X-rays.

United States Patent 1 1 1 ,919,559

Stevens 5] Nov. 11, 1975 l5 l LOUVERED FILM FUR UNIDIRECTIONAL 21333851ui'l938 Freeman 25u5us GHT O A pOlNT SOURCE 1336.026 1211943 Millemmn.ZSWSUS 1435.813 Z/l948 Files r r v r v r v 25Ul5ll8 5] Inventor: EdwardJ- Stevens Luke m 2.5661998 9/1951 Strickmunum 25mins Minn. 173mm W956Schziefer .v ZSWSHS [73] Assignee: Minnesota Mining and M?nuiactul'lng fs Pilul- Prfnuu' limmiucr-Jumes W, Lawrence ASSLSIUHI Emmz'ncr-T. NGrigsb) [221 Fi Ju 12 974 AImr/u). Agent. or Firm-Alexander. Sell.Steldt &

DeLaHunt [2]] Appl. No.: 487,990

Related US. Application Data [63] Continuation-impart of Ser, No.184.403. Aug. 28 [57] ABSTRACT l972 which is a c0ntinuntinwin-purt ofSer No. 2 March mw A process for producing film includingradiationopuque louver-like elements of divergent. or converl l Cl250/503; 161/6 gent. orientation. Pairs of such films for example:[Ill-CL2 1 using radio-opaque louvers. are valuable as Buck [58] Fieldof Search 250/508 505'. 161/6 g id f X-raysr [56] References Cited 10Claims. 10 Drawing Figures UNITED STATES PATENTS 155L162 8/]915 LoebellISWSUX US. Patent Nov.l1, 1975 Sheetlof2 3,919,559

INVENTOR.

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U E Q EDWARD J STEVE/vs US. Patent Nov. 11, 1975 SheetZ 0f2 3,919,559

FIG. 10

INVENTOR. EDWARD J STEVENS BY MWM FIG. 6

1, LOUVERED FILM FOR UNIDIRECTIONAL LIGHT FROM A POINT SOURCE Thisapplication is a continuation-in-part of my copending application, Ser.No. 284,403, filed Aug. 28, 1972, which was a continuation-in-part of myapplication, Ser. No. 128,280, filed Mar. 26, 1971, now abandoned.

This invention relates to a process for the production of film or sheetmaterials having divergent or conver gent radiation-opaque louver-likeelements. lt further relates to the sheet materials produced by theprocess and to laminates of these sheet materials with one another withaxes at different angles with or without cover sheets and to laminatesof single sheets with one or two cover sheets.

It is known from .U.S. Pat. No. 3,524,789 of Olsen that film materialsor sheet materials may be produced including louver-like elementstransverse to the plane of the sheet and substantially normal thereto.Such films with parallel louver elements are known to collimate incidentlight and to be valuable for the control of optical aperture. In thiscase and elsewhere, parallel indicates parallelism of the planes of thelouver elements within i 3 in extremes although usual deviation is muchless, of the order of i 05 to i 2. It is also known to produce film inwhich the louvers are uniformly canted at an angle such thattransparency is only apparent when viewed in a particular direction.Fortuitously, at times, sheet film has been produced in which there wasa gradualchange in angle of cant of successive louverlike elements; butno control has hitherto been possible such that desired ranges of cantor sequences were possible.

For certain purposes, it is desirable that light rays be made divergentor convergent without the use of lenses. A particular problem in thisrespect is with regard to X-rays where it is desired to focus" for thepurpose of giving sharper pictures by absorption of secondary rays. Thefocusing of X-rays by the usual optical lense system is, of course, notfeasible. Handmade devices known as Bucky grids ha ve been produced forsuch purposes. Because these are made with considerable difficul'ty bymanual operations, they are exceedingly expensive, on the order of $100or more per square foot. Such grids are usually mounted between the filmand patient and oscillated slightly during exposure to avoid producing ashadow of the grid in the negative.

It is an object and aim of this invention to provide film materialcapable of divergent or convergent orientation of light and otherradiant energy. Other aims and objects will become apparent hereinafter.

It had been found, in accordance with these objects and aims, thatdifierential distortion of film or sheets having louverlike elementstransverse and normal to the surfaces is possible under conditions suchthat substantially any desired pattern of convergent and/or divergentlouvers is feasible. The procedure is illustrated herein with particularreference to simple converging or diverging sheet material, but it willbe recognized that variations on this procedure can be introduced quitereadily using 'louvers opaque to various wavelengths, variously coloredor combinations of convergence and divergence.

The essential procedure is first, to bond the radiation transparentthermoplastic sheet'containing transverse substantially parallelradiation-opaque louver-like elements between relatively rigid ordimensionally stable, but flexible, cover sheets, most suitably ofmetal, using enough heat to promote adhesion to give a hot laminate,second, to form the still hot laminate produced in the first step arounda curved surface so that the louver-like elements of the originalthermoplastic sheet are parallel to the axis of the curved surface andremain substantially parallel to one another and third, after cooling,separate the deformed thermoplastic sheet from the cover sheets.Flattening means is then applied as a fourth step. ln many cases thestep required is to press the deformed sheet into planarity using heat.ln other cases, where the sheet is relatively very thin, of the order ofabout 1 mm. or less, it may be usable directly as produced because itthen flattens spontaneously to a sufficient extent to give divergence ofthe elements or it may be mounted so as to maintain an essentially flatconfiguration.

Without wishing to be bound by the theory, it would appear that theflattening step may introduce strain which must be relieved by thermalflattening in the case of thicker materials, but is not so great as torequire relief in the case of thinner materials.

In one variation of the invention. the cover sheets are fastened to eachother along one edge so that angle of cant of the louver elements isprogressive from one edge to the other of the light diverging sheet. Itwill be recognized that it is possible to start with a thermoplasticsheet including regularly canted louver elements, that is all canted atthe same angle, and by this procedure superpose further progressivecant. For example, starting with a sheet having 15 cant. a progressivecant of 0 to 15 progressive cant to give one having 0-30 or two suchsheets can be butted to a sheet having +l5 to l5 cant to give acombination of +30 to 30. Such techniques are useful for preparing verylarge sheets which have a close point for convergence of elements aswill be evident.

The final flattened sheet may, in some instances, be used directly or itmay be covered with clear transparent cover sheets on one or bothsurfaces of two or more may be joined together (back to face) with theirlouver elements non-parallel and particularly at right angles, eitherwith or without the use of clear transparent cover sheets. It will, ofcourse, be also possible to employ louver-like elements which arecolored or which have particular properties. A particularly usefulembodiment of the invention is one in which the louverlike elements arecomposed of a radio-opaque substance, e.g., red lead, or powdered leadin a suitable compatible thermoplastic binder and two such divergingsheets are cemented together at right angles to give a screen which canbe employed as a Bucky grid to absorb secondary radiation, i.e.,scattered or stray rays. The range of angles of cant is readilycontrolled so that convergence can be at any desired distance from thescreen. The point of convergence is the point at which the radiantenergy source, i.e. X-ray source, is placed.

It will be recognized that refractive indices of thermoplastic materialsused will affect various radiation differently and visible light willfocus differently from X-rays.

I am aware of the procedures proposed for producing Bucky gridsdescribed in US. Pat. Nos. 2,122,135 and 2,133,385 which rely on thedifficult step of cutting a curved sheet and subsequently flattening it.I am also aware of the disclosures on production of Bucky grids of US.Pat. Nos. 1,551,162, 2,336,926, 2,435,823 and 2,566,998. My procedureusing adhered dimensionally stable sheets is quite different from allthe above.

The initial sheets having transverse radiation-opaque Iouver-likeelements are most conveniently made by the process of the aforementionedU.S. Pat. No. 3,524,789. They may therefore include substantially anyplastic base although cellulose acetate butyrate is a particularlyconvenient one. Polyvinyl butyral is also desirable, but somewhat moredifficult to handle because of its lower melting point. The rigid butflexible cover sheets employed in the first step of the process areusually thin metal which can be bent readily but is not so ductile thatit is stretched under the process of the invention. Suitable commonlyavailable materials include sheets of aluminum of the order of 0.01 to0.04 inches (0.25 mm.) thick. The exact thickness of these sheets is, ofcourse, not critical provided only that they are sufficiently strong towithstand the subsequent operation in which they are employed. Thesurfaces are preferably not glossy when very short focal lengths arebeing produced or when subsequent lamination steps are envisioned. Thesurfaces should never be exceedingly rough. Ferrotype sheets as used inphotography are convenient and useful to provide highly polishedsurfaces. These sheets are not deformed in the process when the radiusof curvature employed is more than about 25 cm. and they may be reused.Usually the radius of curvature is from about 25 to 250 cm. A satinyfinish is quite satisfactory for many purposes.

Lamination of the plastic sheet between the two cover sheets isconveniently carried out in a press at pressures of 25 to 100 psi andabove at temperatures sufficient to soften the thermoplastic polymerinvolved, for example, 300 F. in the case of cellulose acetate butyrate.Suitable padding may be applied on either side of the sandwich orlaminate being made in order to avoid possible adhesion to the platens,to provide greater uniformity of heating and, possibly, to moderate ordistribute pressure more uniformly. The use of padding is not, however,essential to the process of the invention.

The hot sandwich is then deformed by bending over a suitable curvedsurface, for example, a section of a cylinder having a radius of about24 inches. Smaller radii and larger radii are also useful from about 25to 250 cm. Approximately the radius of curvature used is about twice thefocal distance desired. A matching platen may be used or a sheet offabric fastened at one edge of the curved surface may be drawn downtaut. After the polymeric material has cooled, the arcuate thermoplasticsheet separates from the metal cover sheets as the result ofdifferential expansion, that is, due to differences in expansioncoefficients. The cooled sheet is normally somewhat less curved orarcuate than the cylinder around which it was formed to an extentdepending upon a number of factors. The result is that it is notnecessary to have a cylinder of large radius to obtain an arcuatethermoplastic sheet of that radius. When the thermoplastic sheet is ofthe order of 1 mm. or less in thickness, it may fequently be useddirectly because mounting means may exert sufficient force to serve asflattening means but not in the sense of achieving thermal flattening.

The cool arcuate sheet is thermally flattened by application of pressureand heating to about the extent needed for the initial lamination stepand suitably while applying clear cover sheets. The cover sheetsobviously can be colored if desired. This additional flattening step isnecessary for thicker sheet materials of more than about 1 mm. thicknessand optional for thinner sheet materials.

The invention is now further explained by the accompanying drawingswhich show the process of the invention in an essentially diagramaticmanner and also products of the invention.

FIG. 1 shows the hot lamination step to give the hot laminate orsandwich.

FIG. 2 shows placing the hot laminate or sandwich in a press to providecurvature.

FIG. 3 shows the deformation of the laminate or sandwich of FIGS. 1 and2.

FIG. 4 shows that after cooling, the laminate of FIG. 3 separates sothat the metallic sheets are separated. In this and the followingfigures, the arcuate sheet is shown as fully formed around the cylinder.

FIG. 5 shows placing the deformed sheet of FIG. 4 in a heated flatpress, and

FIG. 6 shows the distortion of the louvers after again pressing flat.

FIG. 7 shows a perspective view of the flat sheet of FIG. 6 and FIG. 8shows a top view thereof.

FIG. 9 shows a laminate of two sheets as in FIG. 7 with axes at rightangles and FIG. 10 shows the effect when that laminate is viewed fromabove.

Referring again to the Figures, thermoplastic lightcontrolling sheet 10(suitably about 0.3 to 5 mm. thick) in FIG. 1 having louver elements 18is laminated between 0.50 mm. thick sheets of aluminum 12 by applyingpressure (means not shown) to heated plates 16 having non-adhering pads14. Although some of the desired effect can be obtained by adhering onlyone cover sheet, it is preferred to use two as here described.

The laminate formed may be designated 12-10-12 and in FIG. 2 the stillhot laminate 12-10-12 is placed between arcuate forming means 20,22using adhesion preventing pad (not shown) if desired and pressure isapplied as in FIG. 3 to deform the laminate. Because the angular arcs ofthe upper and lower aluminum sheets 12 are different although the widthsas shown are the same, the apparent effect of this step is the verticaldisplacement of the louver elements 18 although a slight lateral motionof the upper edges may also occur in this step. At the same time as thelaminate is deformed (FIG. 3), it is cooling because no heat is appliedand adhesion of aluminum sheets 12 relaxes and they are freed as shownin FIG. 4 having the arcuate light-controlling sheet 30. It is withinthe scope of the invention to provide heat during the step andsubsequently cool after deformation. As noted above, if the arcuatelight-controlling sheet is sufficiently thin, of the order of 1 mm. orless, it may be used at this point relying on mounting means to providesufficient flattening means to produce substantial divergence of thelouvers.

The arcuate light-controlling sheet 30 is now placed between heatedplatens 16 and non-adhering pads 14 as shown in FIG. 5. The sameapparatus as used in FIG. 1 is shown but, obviously, the exact piece ofequipment is not necessary. Pressure and heat are applied as indicatedin FIG. 6. Because of the way pressure is applied, there are forceswhich deform the arcuate sheet 30 into an optically flat sheet 40 havingthe louver elements variously and progressively inclined as shown.Approximately, the maximum angle of inclination will be a function ofthe angle in FIG. 5 between the lower platen l6 and the tangent to thelower surface of sheet at the outermost edge Clearly. also the smallerthe radius of curvature of sheet 30. the greater will he the maximuminclination of louver elements 18 in sheet 40. The radius of curvaturemay he controlled by the bending means 20, 22 used for deformation inF565. 2 and 3.

FIG. 7 shows a single sheet having convergent tfroni hottom to top)louver elements 18. The axis of the sheet is parallel to these elements.The top view in FIG. 8 shows top edges of louver elements 18 as fulllines and lower edges 52 as hrolten lines. Because the elements areopaque. it will he recognized that from above the sheet 40 will appearmore opaque near the edges and more nearly transparent near the centerwhen viewed at a distance. This view corresponds to any distance otherthan what may he designated the focal length which is the distancealiovc the plane of the sheet at which extensions of all louver elementswould meet. Only at this focal point will the eye see through the entiresheet; at all other positions there will he greater or less obscuringalong the edges.

HG. 9 shows a laminate (which may be further laminatcd with coversheets. not shown. as may also sheet 40 of Flt 7t of two sheets 42 and44 at right angles. l l(i. it) shows a top view but hecause they wouldbe confusing. only upper edges 50 and of louver elements are indicated.The effect is to give a rather square hole in the middle and a focalpoint from which almost complete transparency is attained except forsome distortion along diagonals. A laminate such as shown in Flfji. 10in which the louver elements comprise sufficient radio -opaque materialsuch as red lead serves to focus Xrays as a Bucky grid. Other uses willalso he evident in signals. windows. and other articles depending onoptical properties or effects. Thus. a grid of the type shown in FIGS. 9and 10 may he used for viewing a television or cathode ray screen fromone specific position which would not be visible or only limitedly so.from other positions.

What is claimed is:

1. In a process for the production of thermoplastic sheet materialhaving diverging louver elements. the steps of Ell Ill

(ill

l. thermally bonding at least one thin sheet of metal to a flat sheet ofradiation-transparent thermoplas tic material having substantiallyparallel radiation opaque louver elements at a uniform angle to itssurface to form a hot laminate.

. arcuately deforming said hot laminate forming an arcuate laminatehaving substantially parallel louvcr elements and cooling saidlaminates. and

. separating each said sheet ofnietal from said arcuate laminate aftercooling to provide an arcuate radiationcontrolling sheet havingsubstantially parallel louver elements.

2. lhe process according to claim 1 additional employing the step ofapplying flattening means for llattcning said arcuateradiation-controlling sheet to form a light diverging film wherein thesubstantially parallel louver elements of steps 1). ill and 13) are mademutually divergent.

3. The process according to claim 2 wherein the thermoplastic materialis about I mm. thick or less and flattening means is applied hy mountingso as to maintain an essentially tlat configurationv 4. The processaccording to claim 1 wherein one sheet of metal is bonded to hothsurfaces of the thermoplastic material.

5. The process according to claim 1 wherein one sheet of metal is hondedto each surface of the thermoplastic material.

6. The process according to claim 1 wherein the hot laminate isarcuately deformed in Step [2 l against a cylindrical surface with thelengths of louver elements parallel to the axis of the cylinder.

7. The process according to claim 6 wherein the thermoplastic sheetmaterial is cellulose acetate hutyrate.

8. The process according to claim 1 wherein the louver elements areradioopaquer 9. A radiant energy diverging film of thermoplasticmaterial of about 1 mm. or less in thickness. produced by the process ofclaim 1, and st'iontancously flattened to have divergent radiationcpatue elements.

10. A radiant energy diverging film according to claim 9 havingradio-opaque louver elements. com prised of radio-opaque suhstance andcompatible thermoplastic hinder.

1. In a process for the production of thermoplastic sheet materialhaving diverging louver elements, the steps of
 1. thermally bonding atleast one thin sheet of metal to a flat sheet of radiation-transparentthermoplastic material having substantially parallel radiation-opaquelouver elements at a uniform angle to its surface to form a hotlaminate,
 2. arcuately deforming said hot laminate forming an arcuatelaminate having substantially parallel louver elements and cooling saidlaminates, and
 3. separating each said sheet of metal from said arcuatelaminate after cooling to provide an arcuate radiationcontrolling sheethaving substantially parallel louver elements.
 2. arcuately deformingsaid hot laminate forming an arcuate laminate having substantiallyparallel louver elements and cooling said laminates, and
 2. The processaccording to claim 1 additional employing the step of applyingflattening means for flattening said arcuate radiation-controlling sheetto form a light diverging film wherein the substantially parallel louverelements of steps (1), (2) and (3) are made mutually divergent. 3.separating each said sheet of metal from said arcuate laminate aftercooling to provide an arcuate radiation-controlling sheet havingsubstantially parallel louver elements.
 3. The process according toclaim 2 wherein the thermoplastic material is about 1 mm. thick or lessand flattening means is applied by mounting so as to maintain anessentially flat configuration.
 4. The process according to claim 1wherein one sheet of metal is bonded to both surfaces of thethermoplastic material.
 5. The process according to claim 1 wherein onesheet of metal is bonded to each surface of the thermoplastic material.6. The process according to claim 1 wherein the hot laminate isarcuately deformed in Step (2) against a cylindrical surface with thelengths of louver elements parallel to the axis of the cylinder.
 7. Theprocess according to claim 6 wherein the thermoplastic sheet material iscellulose acetate butyrate.
 8. The process according to claim 1 whereinthe louver elements are radio-opaque.
 9. A radiant energy diverging filmof thermoplastic material of about 1 mm. or less in thickness, producedby the process of claim 1, and spontaneously flattened to have divergentradiation-opaque elements.
 10. A radiant energy diverging film accordingto claim 9 having radio-opaque louver elements, comprised ofradio-opaque substance and compatible thermoplastic binder.